This invention relates to a knock detecting apparatus for detecting knocking of an internal combustion engine.
In a prior art knock detecting apparatus as shown in FIG. 1, components such as vibration and sound generated by knocking of an internal combustion engine are detected by a knocking detector 1 (knock sensor), and a signal A detected by the knock sensor 1 is supplied to an input circuit 2. The input circuit 2 impedance-converts the signal A and passes by way of a filter only a frequency component of the signal corresponding to the knocking frequency. A signal B passing through the input circuit 2 is supplied to one input terminal of a comparator circuit 4. The output signal B of the input circuit 2 is also supplied to a knock discrimination level generator 3. This generator 3 integrates the signal B and forms a reference level signal C for the discrimination of knocking. The comparator 4 compares the signals B and C and yields a knock signal D which is indicative of the existence of knocking, when the signal B is larger than the signal C.
However, an internal combustion engine has many noise sources such as ingition noise, rotation and vibration of the engine. Such a noise is riding on the output line, the power source line, the earth line, etc. Therefore, it is unavoidable that the noise is superimposed on the normal output signal A of the knock sensor 1. Thus, in such a prior art constitution as described above, the output level signal C of the knock discrimination level generator 3 is influenced by noise. The problem is that the knock discrimination level varies with the amplitude of noise and the rotational speed of the engine, and hence no accurate detection of knocking is possible.